The present invention relates to a method and apparatus for monitoring an optical transmission line, and in particular for monitoring the safety of such.
In fibre-optical telecommunications there is an ever present demand for increases in optical data transmission rates and fibre-optical transmission distances. Attempts to meet the former demand typically involve an increase in the number of WDM (Wavelength Division Multiplexed) or DWDM (Dense Wavelength Division Multiplexed) optical channels used to transmit data along optical transmission lines, which typically requires transmission at high optical power levels in order to provide acceptable signal levels in each such channel. Similarly, attempts to meet the later demand requires the use of optical sources and/or optical amplifiers of increasingly high power, and increasingly intense pump sources therefor.
Accordingly, when using such high optical power levels it is preferable to employ a reliable safety mechanism for shutting-down the optical sources and/or optical amplifiers of a telecommunications line should the optical transmission line become broken or disconnected.
The high optical powers present in the transmission line may well be harmful to the human eye at least, and shutdown is generally desirable not only to render the system safe but also to notify of the break itself.
Typically, any acceptably reliable safety mechanism should have an equivalent failure rate no greater than 500 FITs (Failures In 109 hours) in the detection of a break in a transmission line and the subsequent shut-down of the optical source for that line.
Many current safety mechanisms employ an optical splitter positioned on a transmission line at a predetermined point remote from the optical source. The splitter removes from the optical transmission line a small portion of the radiation passing along it. A loop-back optical fibre is connected to the output of the optical splitter and to an associated safety control unit which controls the optical source in dependence upon the loop-back signals being received via the loop-back fibre. If the loop-back signal is lost, as would be the case if either the loop-back fibre breaks or the optical transmission line breaks at a point between the optical source and the splitter, the optical source is shut down.
It will be readily appreciated in requiring a dedicated loop-back optical fibre, safety mechanism such as those described above are relatively expensive to produce in that additional fibres and optical splitters must be provided. Indeed, to implement the system within an N-fibre optical cable would require dedicating N/2 of those fibres to looping back the signals carried upon the other N/2 fibres. Thus, only half of the fibres within the cable would be available for data transmission.